In vehicles, in particular motor vehicles, disk brakes are by far the most common type of brake system. Disk brakes generally include a brake disk and a brake caliper surrounding the brake disk on the edge. The brake disk is connected to the wheel of the vehicle to be braked by a wheel hub mounted rotatably in the stub axle. In contrast, the brake caliper is fixed to the stub axle. The actual deceleration is achieved by brake pads which may be placed against on the brake disk and are arranged on both sides of the disk between it and the brake caliper.
Depending on application, brake disks may include iron or also of carbon ceramic or aluminum. Brake disks often have a surface which is as wear-resistant and emits as little dust as possible. To achieve this, it is desirable that the surface be as hard as possible. For this reason, brake disks with a metallic base body are often fitted with a suitable coating. The coating extends at least over an annular surface region of the base body which serves as a friction face or friction ring for the brake pads. This is normally a flat surface region which is composed of at least two friction faces extending parallel to each other and defining the friction ring at the sides and hence in its thickness.
Since brake disks are mass-produced consumable items, these are made primarily of iron, in particular cast iron. Without the provision of a coating, iron oxide would form rapidly. This is a corrosive process which may destroy the brake disk over time. As well as the deterioration in appearance caused by the light rust film, the formation of surface rust leads in many cases to an acoustic deterioration which is expressed in an unpleasant squealing.
A suitable coating may for example be achieved with thermal spraying. Here the material to be applied to the surface of the base body is first softened by the effect of heat and accelerated in the form of individual particles through a gas flow. When the particles hit, a purely mechanical connection is created without melting the surface of the base body. These materials may be metals or oxide-ceramic or carbide materials.
In order to improve the connection between the coating and the surface region, the surface region is first roughened. Pure sandblasting or corundum blasting achieves only a moderate bonding of the coating to the surface region of the base body.
DE 10 2011 075 821 A1 describes a brake disk and a method for its production. The brake disk includes a base body, on the surface of which a wear-protection layer is applied, for example by high velocity oxyfuel flame spraying (HVOF) and/or atmospheric plasma spraying (APS). To improve the connection between the surface of the base body and the wear protection layer to be applied, the surface is suitably pretreated. For this the use of laser beams is proposed, to obtain a suitable surface topography of the base body. A suitable surface topography is for example the application of a meandering and/or groove-like and/or spiral structure.
The roughening of the surface region which may be obtained by laser radiation is better than that of sand- or corundum-blasting. However here again, sometimes an insufficiently durable connection is achieved between the base body and the coating. The reason for this is the absence of undercuts which achieve a mechanically strong bond. In order to create such advantageous undercuts, various further methods are known in the prior art.
DE 10 2010 064 350 A1 discloses a brake disk with a coating. In this context, a method is shown with which the coating may be applied to a surface of the brake disk. In order to increase the adhesion strength between the brake disk and the coating, the surface of the brake disk is fitted with recesses. The side walls defining the recess are configured such that a width of the recess increases with the depth. As a result, at least one side wall of the recess runs at an angle in relation to a surface of the brake disk so that at least one undercut is produced. These recesses are produced in the surface of the brake disk by material removal, for example via a tool with corresponding cutting edges. The undercut allows a bonding of the coating to be applied onto the surface of the brake disk and in particular into its recesses.
Furthermore DE 10 2006 045 275 B3 describes a method for preparation of a surface to be coated on a base body of a product. To achieve a high adhesion of the coating to the base body, its surface is machined by material removal before application of the coating. The resulting recess is configured such that the side walls defining the recess are produced at an angle to each other. The recess tapers from its base towards the surface, whereby the opposing side walls of the recess each form an undercut (dovetail). To achieve this double undercut, a material-removing tool is disclosed which has two opposing cutting surfaces oriented correspondingly to each other.
According to DE 10 2006 004 769 A1, to improve the adhesion of thermally sprayed layers on a metal surface, it should first be roughened. For this the arrangement of indentations or recesses is proposed which are formed by material removal or chipping. The parts of the metal surface remaining between the recesses or indentations form corresponding projections, fluting, bulges or embossing, which in the next step are mechanically formed or broken down. The aim is to form a plurality of undercuts with the parts of the metal surface extending into the respective recesses or indentations. This achieves a mechanical bonding between the prepared metal surface and the layer thermally sprayed onto this. Alternatively the raised parts may be melted by the effect of heat, and after setting, the melted droplets form the undercuts. Also a material-removing machining of the raised parts of the metal surface is disclosed, which includes an only incomplete metal detachment. As a result additional undercuts are created which are formed by partly bent or folded parts of the machined metal surface. To bend the raised parts of the metal surface, in particular a rolling process is proposed, or an obliquely applied pressing or blasting method, for example with a fine round powder, and sand-blasting, corundum-blasting or bead-blasting. In the rolling process it is considered advantageous if the roughness (Rz value) of the metal surface is reduced by the rolling.
DE 10 2010 052 735 A1 discloses a brake disk and a method for its production in which a brake disk base body is given a thermal spray coating. To improve the adhesion of the spray coating, here again a suitable superficial preparation is taught. This includes the arrangement of at least one recess groove in the base body of the brake disk. If several recess grooves are made in the brake disk base body by means of a material-removal process, the grooves are arranged in the form of a spiral or concentric circles. At least one of the walls defining the recess grooves is configured such that this extends at least partly into the recess groove in order to form at least one undercut in the recess groove. In addition, immediately after production of the recess grooves, a forming process is carried out in which the parts defining the recess grooves are broken/formed to create the undercuts. To produce the recess grooves and form the remaining surface parts, it is proposed to clamp the brake disk base body in a lathe. During its notation, a cutting tool and a forming tool are guided, starting on a first machining radius of the brake disk base body, radially outward or inward over its surface. The forming process is carried out directly after the cutting process to produce the recess grooves. A suitable tool is a dual plate which includes a cutting and a forming plate. Thus after production of the recess groove by the cutting plate, immediately afterwards the forming plate of the dual plate is used for the necessary forming step. Finally the prepared surface of the brake disk base body is spray-coated.
DE 10 2011 087 575 A1 describes a method for production of a composite component coated with a friction layer. The composite component may be a brake disk. However the composite component has pin-like raised parts formed as protrusions, which are produced during the process of casting the surface to be coated. The pin-like protrusions are distributed equidistantly from each other over the surface to be coated. Grooves which are cut into the base body by material removal are not disclosed in DE 10 2011 087 575 A1. However DE 10 2011 087 575 A1 also discloses undercuts at the pin-like protrusions. For this the undercuts are made approximately rotationally symmetrically to the longitudinal center axis of the pin-like protrusion and have a mushroom head form. The undercuts are produced by plastic deformation, e.g. by flattening or rolling, like in the method known for riveting, so that the head mushrooms under force.
DE 10 2012 207 455 A1 is concerned with a tool for mechanical roughening of an inner surface of a bore and with a corresponding method using this tool. In particular cylinder bores of internal combustion engines may be prepared suitably to allow their subsequent thermal coating. For preparing the surface, a combination of material removal and rolling is disclosed. First a profiled cutting plate is used with which linear recesses are made in the surface of the substrate. Profile webs remaining between the recesses are then rolled with a roller. The pressure exerted on the profile webs by the roller causes the profile webs to be reduced by up to 50% of their initial height. The cylindrical roller used for this has diamond grains arranged on its casing surface. The cylindrical roller has a constant diameter over its extension. After rolling, the edges of the profile webs are deformed such that material overhangs and micro cracks occur in the form of undercut-like and hence form-fit contours. The diamond grains create crater-like indentations on the heads of the profile webs. Depending on application, the roller is guided such that this either just rolls on the profile web or slides on this due to a non-orthogonal axial position to the course of the profile web. In the latter case, as well as indentations, additional scoring is produced which is attributable to the material-removal effect of the diamond grains partly sliding on the profile web. Also DE 10 2012 207 455 A1 discloses a possible conical design of rollers which, as they pass increasingly axially over the machining point, build up an increasing pressure and hence cause an increasing deformation of the webs.
DE 20 2009 014 180 U1 is concerned with cylinder bores in which coatings are applied thermally which, after final machining, have certain desirable properties in relation to friction and wear. A material-removal process to produce profile webs is combined with a rolling process for forming the profile webs. The forming tool has a profiling. The profiling may be formed as triangular peaks, wherein however also a profile in the form of a recessed curvature may be provided. It is furthermore proposed to allow two profiled forming tools to follow each other. A first forming tool has a notch in which the following forming tool engages to produce the final profile.
DE 10 2009 058 178 A1 discloses a method for surface treatment of cylinder bores in which firstly, recesses and bulges are produced by means of material-removal machining. Undercuts are produced by forming the bulges, wherein a spray coating is applied to the surface. The principle of recess swaging is used for forming, wherein the tappets serving as a forming element of a swaging machine are tapped radially outward against portions of the bulges of an inner face of the hollow cylinder.
The known methods allow the formation of undercuts on at least one surface region of the brake disk base body. However the production of recesses with at least one sloping side wall to form the undercut is still complex. The cutting tools required for this often must have a corresponding contour or setting of their constituent parts. The tools, which are already costly to produce, are also subject to high wear. This also applies to the arrangement of a double tool in which the cutting plate is followed by a forming plate. The contact pressure necessary for deforming the remaining parts of the surface region is also associated with a high deceleration rate of the base body which is usually clamped rotatingly. Any catching at a change of surface region, which is normally difficult to control, may lead to disadvantageous contour courses.